Liquid developer used for electrophotography

ABSTRACT

A LIQUID DEVELOPER FOR DEVELOPING AN ELECTROSTATIC LATENT IMAGE COMPRISING A CARRIER LIQUID HAVING MORE THAN 10**11$ CM. OF SPECIFIC RESISTANCE, TONER PARTICLES WHICH ARE INSOLUBLE IN SAID CARRIER LIQUID, 1-HYDROXYALKYL-2-HIGHER ALKYL-2-IMIDAZOLINE WHICH IS SOLUBLE IN THE CARRIER LIQUID AND A COPOLYMER OF HALF-ALKYLAMIDE OF MALEIC ACID AND DIISOBUTYLENE WHICH IS SOLUBLE IN THE CARRIER LIQUID IS DISCLOSED.

United States Patent US. Cl. 252--62.1 Claims ABSTRACT OF THE DISCLOSURE A liquid developer for developing an electrostatic latent image comprising a carrier liquid having more than 10 0 cm. of specific resistance, toner particles which are insoluble in said carrier liquid, l-hydroxyalkyl-Z-higher aIkyI-Z-imidazoline which is soluble in the carrier liquid and a copolymer of half-alkylamide of maleic acid and diisobutylene which is soluble in the carrier liquid is disclosed.

BACKGROUND OF THE INVENTION (1) Field of the invention The present invention relates to a developer used for developing electrostatic latent images and more particularly to an improved liquid developer for converting electrostatic latent images into visible images in the electrophotographic or electrostatic printing processes. This improved developer is especially useful when developing electrostatic latent images on an insulating surface by the use of toner particles which are charged positively.

(2) Description of the prior art In the electrophotographic process, a surface of a recording element, which typically comprises a sensitive photoconductive zinc oxide coating on a support having a comparatively high conductivity, is charged uniformly negatively in darkness.

An optical image of an original image is projected to the charged surface of the sensitive layer, whereby the uniformly charged surface is discharged imagewise corresponding to the light intensity and an electrostatic latent image is formed. By applying electroscopic toner particles to this electrostatic latent image, a visible image is produced. In the Electrofax process, this visible image is fixed directly to the photoconductive surface. It is necessary when copying ordinary documents to obtain a positive reproduction from an optically positive original. Accordingly, when the surface of the sensitive layer is charged negatively, the electroscopic toner particles should have a strong and stabilized positive charge. Today many kinds of liquid developers including toner particles of positive polarity have been marketed. However, these commercially available developers are designed for copying a line image original or a dotted image original; consequently, they are not suitable for reproduction of an image of continuous gradation. When a continuous gradation image is reproduced using these developers, a sufficient image density is not obtained, a so-called streak is easily formed, and deposition of the toner on background areas causes what is referred to as fog.

SUMMARY OF THE INVENTION The liquid developer of the present invention is characterized in that it can produce a high image density. Further, the developed image is substantially free of streaks and background toner deposition (fog). Furthermore, the toner particles contained in the liquid developer of the present invention are stable over extended periods 3,772,199 Patented Nov. 13, 1973 of time as concerns the dispersion and the electrophoretic properties.

DETAILED DESCRIPTION OF THE PRESENT INVENTION The liquid developer of the present invention is characterized by containing l-hydroxyalkyl-Z-higher alkyl-2- imidazoline and a copolymer of half-alkylamide of maleic acid and diisobutylene dissolved in a carrier lipuid.

Either of these two compounds have an influence upon electrostatic atmosphere of the toner imparting a positive charge to the toner in a high insulating carrier liquid (hereinafter referred to as the carrier liquid). This effect is more intense with the use of the copolymer of half-alkylamide of maleic acid and diisobutylene (hereinafter referred to as Imidazoline A). It has been found that these two compounds when used together exhibit a more intense ability of charge control on the toner than when used separately, and that, further they show a similar synergistic effect on the stabilization of the dispersion of the toner.

In the present invention well-known inorganic and organic pigments are used as the toner. Suitable toners include carbon black, Aniline black, Crystal Violet, Phthalocyanine Black, Phthalocyanine Green, Molori Blue, Hausa Yellow, Quinacrydone Red, Quinacrydone Violet, Phthalocyaniue Blue, Quinacrydone Magenta and Benzidine Yellow.

These pigments are prepared as a concentrated pigment paste, namely a concentrate of toner particles. An improved liquid developer can be produced by diluting the concentrated pigment paste with the carrier liquid. The pigment paste of toner particles is admixed with Imidazoline A, Copolymer B, and if desired, resins which are soluble in the carrier liquid. When a small amount of a carrier liquid is admixed with this admixture, i.e. the pigment paste, it is capable of easily providing a developer having a desired concentration which is suitable for the specific use by adding a suitable amount of the carrier liquid at use. It is desirable that the particle size of the toner suitable as the developer of the present invention be less than 5 microns and preferably less than 1 micron so as to obtain a high quality continuous tone image.

It is preferable that the hydroxyalkyl group in the lhydroxyalkyl 2 higher alkyl 2 imidazoline (Imidazoline A) used in the present invention contain 1 to 7 carbon atoms and preferably be hydroxyethyl, hydroxypropyl or hydroxybutyl. Further, it has been found that those higher alkyl groups having 6-22 carbon atoms as the higher alkyl group are preferable for the purpose of the present invention in the viewpoint of charge controlling ability and solubility in the carrier liquid.

In the present invention, a copolymer of half-alkylamide of maleic acid and diisobutylene is used together with the above-mentioned imidazoline A. It has also been found that the half-alkylamide having an alkyl group of 24 carbon atoms and having a molecular weight of 1000- 50,000 is preferable for the purpose of the present invention, since it has good solubility in the carrier liquid ordinarily used and excellent charge controlling ability.

The molar ratio of monomer in the copolymer may be varied in a wide range, but copolymers having a ratio of half-alkylamide of maleic acid and diisobutylene of about 1:1 to about 1:5 are preferably used.

The carrier liquid used in the improved developer of this invention may be a hydrocarbon solvent, for example, cyclohexane, Decalin, kerosene, isoparaffin, light oil and gasoline, etc. Other carrier liquids suitable for use include chlorofluorinated hydrocarbon solvents and silicone oils which can be used solely or as a mixture. The liquid should have a specific resistance of more than 10 0 cm. and preferably more than 10 82 cm.

In the present invention, resins which are soluble in the carrier liquid may be used as dispersion promoting agents. The resins which serve this purpose include alkyd resins which are well-known in the field of this technology, styrene polymers, rosin, asphalt, and polybutene; many vegetable oils can be used also. Further, a small amount of additives such as saturated fatty acids, unsaturated fatty acids, salts thereof and anti-oxidants, etc., may be added as is typical in the art.

As one method of the preparation of the developer, a toner may be blended with a resin, Imidazoline A and Copolymer B. The mixture is then diluted by a carrier liquid in a suitable amount to produce a developer having a predetermined toner concentration. In another method, a toner is kneaded initially with a resin and then the mixture is diluted with a carrier liquid containing Imidazoline A and Copolymer B. In short, time of addition of Imidazoline A and Copolymer B to the liquid developer can be selected freely.

A preferable toner concentration is 0.1-50 parts by weight based on 1000 parts of the liquid carrier. When the toner concentration is too low, image density tends to be insufiicient in a short development period and when the toner concentration is too high, background toner deposition becomes great.

Since the Copolymer B has a high charge controlling ability, good effect is brought about with the addition of 0.0005 part of this compound based on 1000 parts of the carrier liquid. An excellent result is brought about by the addition of 0.00l-0.05 part of Copolymer B based on 1000 parts of the carrier liquid. Addition of the Copolymer B in the amount beyond this range causes lowering of the image density.

Imidazoline A is preferably used in the amount of Ma- 20 times (by weight) of the amount of Copolymer B. But the allowable maximum amount of Imidazoline A is 0.5 part based on 1000 parts of the carrier liquid. Addition of the Imidazoline A in an amount beyond this range is apt to bring about lowering of the image density.

The resins which are soluble in the carrier liquid may be added in the amount of one to 50 times the amount of the toner. However, it is preferable that the amount does not exceed 100 parts based on 1000 parts of the carrier liquid. Addition of the resins is not necessary when using a grafted toner which comprises a pigment and polymer chains chemically bonded onto the pigment surface, such as described in Belgian Pat. No. 706,742 and US. Pat. No. 3,503,881. This type of developing agent comprises a reaction product produced by a graftpolymerization to a pigment such as carbon black of an alkyl ester of acrylic or methacrylic acid, and on occasion, further of a small amount of polar monomer such as acrylic acid, acrylic amide, etc. (One commercially available example of such a product is Graft carbon" marketed by Japan Gas-Chemical Co., Inc.) The developing agent of this type is added to a carrier liquid together with the imidazoline and the copolymer of the present invention to form a liquid developer having the above-mentioned superior properties. In this case it is not necessary to add the resin soluble in the carrier liquid to the carrier liquid.

The upper limit of the sum of various additives added to the liquid developer of the present invention depends mainly to what extent the electrical resistivity of the resulting developer is lowered. To obtain a high quality continuous tone image the liquid developer should show an electrical resistance not less than 5 10 n cm. when measured after the removal of the dispersed particles. Thus, it is necessary to control the amount of each additive. In order to remove the toner from the developer, an electrophoretic process is preferably utilized. By removing the toner by the electrophoretic process, the pigment dispersed and additives strongly adsorbed on or associated with the pigment surface are removed. The remaining liquid phase which consists of a carrier liquid A concentrated pigment paste of the following formulation was prepared which could be diluted with a carrier liquid to form a liquid developer.

Parts Carbon black 10 Copolymer of half-docosanylamide of maleic acid and diisobutylene (degree of polymerization:

l-hydroxyethyl-2-dodecyl-2-imidazoline 0.2

Satflower oil modified alkyd resin (oil length:

Isoper-H (Isoparafiinc solvent produced by. Esso Standard Oil Co.)

These ingredients were mixed in a ball mill for 40 hours to produce a mixture having good dispersibility. (In order to obtain good dispersibility, means such as a high speed mixer, a colloid mill and a vibration mill can also be used.) The resulting concentrated paste was diluted with a carrier liquid to produce a liquid developer. 30 parts of this concentrated paste was diluted with 1000 parts of Isoper-H.

On the other hand, a surface of an electrophotographic zinc oxide sensitive layer was charged uniformly by a corona electrode applied to 7 kv. in the dark. Then, the layer was exposed to light through a positive original having a continuous gradation to form an electrostatic latent image thereon.

After being wetted at the surface bearing the electrostatic latent image with Isoper-H not containing any toner, the sheet was immersed in the liquid developer described above for 30 seconds, using a development electrode. The surface of the sensitive layer after development was washed with fresh Isoper-E (Isoparaffin type solvent produced by Esso Standard Oil Co.). Thus, a positive reproduction of the continuous tone image could be obtained. The maximum optical reflection density of the image was 2.24 and the minimum (fog density) was 0.07. Streaks of tone were hardly observed in the image. When the same procedure was repeated again after the preservation of the developer for 50 days at room temperature, a positive image having a maximum optical density of 2.20 and a minimum 0.07 was obtained. No substantial change was observed as for the dispersion condition of the toner. The image exhibited a somewhat warm black appearance.

COMPARATIVE EXAMPLE 1 A liquid developer was produced by the same procedure as in Example 1, but excluding 1-hydroxyethyl-2-decyl-2- imidazoline. Development was carried out by the same procedure as in Example 1. A positive image having an optical density of a maximum of 2.20 and a minimum of 0.09 was obtained. Slight tendency of streaking was observed in the image. When developed at the same conditions after being preserved for 50 days at room temperature, a positive image having a maximum optical density of 1.78 and a minimum of 0.10 was obtained. Precipitation of a small amount of the toner was observed on the bottom of the container.

COMPARATIVE EXAMPLE 2 A liquid developer was prepared by the same procedure as in Example 1, but only excluding the copolymer of half docosanylamide of diisobutylene. Then development EXAMPLE 2 The copolymer of half-dooosanylamide of maleic acid and diisobutylene was replaced by half-octadecylamide in the preparation of a developer as described in Example 1. When developed by the same procedure as in Example 1, an image having a maximum optical density of 2.12 and a background, or minimum density of 0.08 was obtained.

EXAMPLE 3 A liquid developer was prepared using a mixture of 0.15 part of 1-hydroxyethyl-2-dodecyI-Z-imidazoline and 0.005 part of 1-hydroxyethyl-2-stearyI-Z-imidazoline instead of 0.2 part of 1-hydroxyethyl-2-dodecyl-2-imidazoline used in Example 1. With the development according to the same procedure as described in Example 1, a substantially equivalent result was obtained.

EXAMPLE 4 9 parts of carbon black and 1 part of Phthalocyanine Blue were used as a pigment mixture instead of parts of carbon black used in Example 1. By practicing the same procedure as in Example 1, an image having a slightly blue-black appearance resulted.

EXAMPLE 5 12 parts of Phthalocyanine Blue were used instead of 10 parts of carbon black used in Example 1. By practicing the same procedure, a blue image having little streaking was obtained. The maximum image optical density was 2.04 and the background density was 0.10.

EXAMPLE 6 A liquid developer having the following composition was prepared.

Parts Reaction product obtained by graft polymerization of 5 parts of carbon black and 7 parts of lauryl methacrylate 2 Copolymer of half-docosanylamide of maleic acid and diisobutylene 0.05 1-hydroxyethyl-2-dodecyl-Z-imidazoline 0.1 Isoper H 1000 An excellent black image without fog was obtained using this developer by the same procedure as described in Example 1. After 60 days of storage, the superior electrical properties and dispersing stability of the developer were maintained.

What is claimed is:

1. A liquid developer for developing an electrostatic latent image comprising a carrier liquid having more than 10 0 cm. of specific resistance, toner particles of diameter less than 5 microns which are insoluble in said carrier liquid, l-hydroxyalkyl-Z-higher alkyl-Z-imidazoline which is soluble in said carrier liquid, said hydroxyalkyl containing from 1 to 7 carbon atoms and said higher alkyl containing from 6 to 22 carbon atoms, and a copolymer of half-alkylamide of maleic acid and diisobutylene which is soluble in said carrier liquid, wherein said toner is present in an amount from about 0.1 to 50 parts by weight per 1000 parts of said carrier liquid, said copolymer is present in an amount from about 0.0005 to 0.05 part by weight per 1000 parts of said carrier liquid and the weight ratio of the imidazoline compound to said copolymer is in the range of about A; to 20.

2. A liquid developer as claimed in claim 1 wherein the hydroxyalkyl group of said imidazoline is selected from the group consisting of hydroxyethyl, hydroxypropyl and hydroxybutyl.

3. A liquid developer as claimed in claim 1 wherein the alkyl group in the alkylamide group of said copolymer contains 6 to 24 carbon atoms and the molecular weight of said copolymer is between 1,000 and 50,000.

4. A liquid developer for developing an electrostatic latent image comprising a carrier liquid having more than 10 0 cm. of specific resistance, a pigment, a resin which is soluble in the carrier liquid, 1-hydroxy-alkyl-2-higher alkyl-2-imidazoline which is soluble in said carrier liquid, said hydroxyalkyl containing from 1 to 7 carbon atoms and said higher alkyl containing from 6 to 22 carbon atoms, and a copolymer of half-alkylamide of maleic acid and diisobutylene which is soluble in said carrier liquid, wherein said copolymer is present in an amount from about 0.0005 to 0.0 5 part by weight per 1000 parts of said carrier liquid and the weight ratio of the imidazoline compound to said copolymer is in the range of about A; to 20.

5. A liquid developer for developing an electrostatic latent image comprising a carrier liquid having more than 10 0 cm. of specific resistance, a graft carbon type developing agent, l-hydroxyalkyl-Z-higher alkyl-2-imidazoline which is soluble in said carrier liquid, said hydroxyalkyl containing from 1 to 7 carbon atoms and said higher alkyl containing from 6 to 22 carbon atoms, and a copolymer of half-alkylamide of maleic acid and diisobutylene which is soluble in said carrier liquid, wherein said copolymer is present in an amount from about 0.0005 to 0.05 part by weight per 1000 parts of said carrier liquid and the weight ratio of the imidazoline compound to said copolymer is in the range of about to 20.

6. A liquid developer as claimed in claim 4 wherein said resin is present in an amount from about 1 to parts by weight per 1000 parts of said carrier liquid.

7. A liquid developer as claimed in claim 1 wherein said toner particles have a diameter of less than 1 micron.

8. A liquid developer as claimed in claim 1 wherein said carrier liquid is a member selected from the group consisting of cyclohexane, Decalin, kerosene, isoparafiin, light oil, gasoline, chlorofiuorinated hydrocarbon solvents and silicone oils.

9. A liquid developer as claimed in claim 1 wherein said imidazoline compound is 1-hydroxyethyl-Z-dodecyl-Z-imidazole and said copolymer is a copolymer of half-dooosanylamide of maleic acid and diisobutylene having a degree of polymerization of 6.

10. A liquid developer as claimed in claim 5 wherein said graft carbon type developing agent is the reaction product obtained by graft polymerization of 5 parts of carbon black and 7 parts of lauryl methacrylate.

References Cited UNITED STATES PATENTS 3,554,946 1/ 1971 Akuno et a1. 252-62.1 3,503,881 3/ 1970 Shinokara et a1 25262.1 3,079,272 2/ 1963 Greig 2526 2.1 3,244,633 4/ 1966 Yellin et a1. 252-62.1

NORMAN G. TORCHIN, Primary Examiner J. P. BRAMMER, Assistant Examiner 

